This invention relates generally to water quality management and more particularly to a system and method for substantially preventing sewer solid accumulation in an urban drainage system.
Most urban drainage systems have evolved into a complex network that includes combined sewer systems (including interceptor sewers), separated sanitary sewer systems, stormwater sewer systems, channels, and culverts. This network conveys domestic and industrial wastewater to wastewater treatment plants during dry weather (referred to as xe2x80x9cdry weather flowxe2x80x9d) with the addition of stormwater runoff during periods of wet weather (collectively referred to as xe2x80x9cwet weather flowxe2x80x9d). Domestic wastewater includes sewage from a household. Industrial wastewater includes industrial processing waste including solids and liquids.
A xe2x80x9ccombined sewer systemxe2x80x9d collects domestic and industrial wastewater, and stormwater runoff. This mixture is called combined sewage. A xe2x80x9cseparated sanitary sewerxe2x80x9d collects domestic and industrial wastewater. A xe2x80x9cstormwater sewer systemxe2x80x9d collects stormwater. During dry weather or small rainstorms, combined sewage from combined sewer systems and wastewater from sanitary sewer systems receive full treatment before discharge to receiving waters. During larger rainstorms, inflows can exceed the capacity of these sewer systems or the wastewater treatment plant itself. The excess flows are known as combined sewer overflows (CSOs) and sanitary sewer overflows (SSOs) xe2x80x9cWet weather flowxe2x80x9d discharges include combined sewer overflows, sanitary sewer overflows, and stormwater runoff. Dry and wet weather flow include both sewer solids and liquid. Combined sewer overflows, stormwater runoff, and sanitary sewer overflows are major contributors to the degradation of many urban lakes, streams and rivers.
CSOs and SSOs may be diverted respectively to CSO and SSO storage tanks to substantially reduce or eliminate the frequency and volume of CSOs/SSOs to receiving waters. These storage tanks are located in order to intercept the CSOs/SSOs before they enter the receiving waters. They store the excess wet weather flow during rainstorms. Stormwater storage tanks similarly store excess stormwater during rainstorms. During this period, sewer solids in the wet weather flow settle to the bottom of the tank. When flows subside after a rainstorm, their liquid contents are drained or pumped back into the appropriate sewer systems and conveyed to the wastewater treatment plant where they are treated. After the liquid contents of the tank are emptied, the settled solids remain on the floor of the tank. Sewer solids deposited in combined sewer and sanitary sewer systems during low flow dry weather periods are major contributors to the CSO/SSO-pollution load, causing serious water quality and health problems.
One of the underlying reasons for considerable sewer solids deposition is the combined sewer hydraulic design. Combined sewers are sized to convey many times the anticipated peak dry weather flow. Combined sewers can carry up to 1000 times the expected background sewage flow. Ratios of peak to average dry weather flow usually range from 2 to 10 for interceptor sewers. The oversized combined sewer segments possess substantial sedimentation potential during dry-weather periods. Dry weather flow velocities are typically inadequate to maintain settleable solids in suspension, and a substantial amount of sewer solids tends to accumulate in the pipes. During rain storms, the accumulated solids may resuspend and, because of the limited hydraulic capacity of the interceptor sewers, overflow to receiving waters. Suspended solids concentrations of several thousand parts per million are not uncommon for CSOs. This can produce shock loadings detrimental to receiving waters. Accumulation of sewer solids in sewer pipes also results in a loss of flow carrying capacity that may restrict/block flow and cause an upstream surcharge or local flooding.
Sewer solid accumulation in urban drainage systems also creates septic conditions that pose odor, health hazards, and corrosion problems for these systems. xe2x80x9cSewer solidsxe2x80x9d as used herein may include sediment, sludge, debris or the like. xe2x80x9cCombined sewer systemxe2x80x9d as used herein includes both combined sewers and CSO storage tanks. xe2x80x9cSanitary sewer systemxe2x80x9d as used herein includes both sanitary sewers and SSO storage tanks. xe2x80x9cUrban drainage systemxe2x80x9d as used herein includes combined sewers, sanitary sewers, stormwater sewers, and CSO/SSO/stormwater storage tanks.
A variety of flushing systems have been used to purge the sewer solids deposited in combined sewers, stormwater conveyance systems and CSO storage tanks. By creating high-speed flushing waves to resuspend deposited solids, the resuspended solids are washed to strategic locations such as to a point where the wastewater stream is flowing with sufficient velocity, to another point where flushing will be initiated, to a storage sump which will allow later removal of the stored contents, or the wastewater treatment plant. Flushing reduces the amount of solids resuspended during storm events, lessens the need for CSO treatment and sludge removal at downstream storage facilities and allows the conveyance of more flow to the wastewater treatment plant or to the drainage outlet.
One such system is the Hydrass(copyright) flushing system comprised of a balanced hinged gate. The gate is weighted to close during low flows allowing the flow to be retained behind the gate. Once the force created by the retained water becomes sufficient, the gate tilts. This releases the surcharged water and flushes the sediment from the sewer. Once the force of the surcharged water is relieved, the gate returns to the closed position to repeat the line surcharging.
Another system is the Hydroself(copyright) flushing system which uses a storage impoundment to retain water. Periodically this water is released creating a hydraulic surge which flushes deposited sediment from the storage tank floor and along sewer lines. The release can be triggered manually or automatically with a preset water level monitor and controller.
The gate flushing system also requires a storage impoundment for the flush water. This is created by erecting two walls in the sewer pipe. A heavy gate is placed in the sewer or storage tank perpendicular to the flow and water is held behind the gate. When the water level behind the gate reaches a predetermined level, the heavy gate is opened and water is released to flush sediment downstream of the gate. The impoundment floor must have a slope of 5 to 20% to prevent debris accumulation. When the water reaches a predetermined level, it is released causing a hydraulic surge that flushes the storage tank and sewer line.
The tipping flushers system uses a cylindrical stainless steel vessel suspended above the maximum water level on the back wall of the storage tank. The system requires a water filling system. As the vessel is filled with water, the center of gravity shifts and causes the vessel to rotate and discharge its contents down the back wall of the tank. A curved fillet at the intersection of the wall and tank floor redirects the flush water horizontally across the floor of the storage tank. The flushing force removes the sediment and debris from the tank floor and transports it to a collection sump located at the opposite end of the tank. These flushing systems all require an extramural source of water and/or complex control instrumentation.
Accordingly, there has been a need for a novel system and method that substantially removes sewer solids from urban drainage systems between storms. There is also a need for a novel system and method that may be used in urban drainage systems for substantially reducing sewer solids and associated pollutants from reaching receiving waters. There is a still further need for a novel system and method that operate under atmospheric pressure and hydrostatic head build-up. There is an additional need for a novel system and method that do not require an extramural source of water for flushing. There is a still further need for a novel system and method that do not require complex control instrumentation. There is an additional need for a novel system and method that is cost effective. The present invention fulfills these needs and provides other related advantages.
In accordance with this invention, the system comprises, generally, at least one flush reservoir within an urban drainage system, the at least one flush reservoir having an ingress and egress port therein through which wet weather flow is received from and discharged in a surge to the urban drainage system, an air intake conduit for drawing air into the at least one flush reservoir, and an air release valve that closes when the at least one flush reservoir is substantially full to create a vacuum on draining of the urban drainage system, the vacuum breaking when the urban drainage system is drained to a level permitting the intake of air to break the vacuum in the flush reservoir, thereby discharging the wet weather flow from the at least one flush reservoir to flush accumulated sewer solids from the urban drainage system.
The at least one flush reservoir defines a box-like receptacle having a top portion and downwardly-extending sidewalls. The floor of the flush reservoir is the floor of the CSO/SSO/stormwater storage tank or sewer line flush chamber in which the at least one flush reservoir may be installed.
The ingress and egress port may be provided in one of the sidewalls along the bottom edge thereof. The flush reservoir opens to the sewer line flush chamber or storage tank through the ingress and egress port. The height of the port is about two to about four inches greater than the historical height of the sediment (sewer solid) layer.
The air intake conduit may extend from an upper opening in the flush reservoir to a lower opening along a sidewall other than the sidewall with the ingress and egress port. The air intake conduit may be in the form of a rectantular duct defined by a partition wall or in the form of an air intake tube connected to the flush reservoir at the upper opening by a tee joint. The lower opening may be sized to be about thirty percent of the size of the ingress and egress port. The lower opening may be about two to three inches higher than the top of the ingress and egress port.
The air release valve for the at least one flush reservoir is installed through the top of the flush reservoir above the maximum level of wet weather flow in the flush reservoir. The air release valve may be a check valve that permits the release of air from the at least one flush reservoir when it is filling with wet weather flow.
The at least one flush reservoir may be installed in an upstream end of the storage tank and/or sewer line with the ingress and egress port facing the downstream end of the storage tank or sewer line flush chamber. The ends of the flush reservoir may be mounted to the floor of the storage tank or sewer line flush chamber. When installed in the CSO/SSO/stormwater storage tank or sewer line, the volume of the flush reservoir may be about 10-20 percent of the volume of the storage tank. For sewer line applications, the flush reservoir volume may be about 20-50% of the volume of the total length of the sewer line to be flushed. The at least one flush reservoir may be sized to fit through the manhole for installation in the sewer. Preferably, there is at least one flush reservoir for every 500-1000 feet of sewer.
In use during a storm, when the storage tank or sewer line flush chamber downstream of the flush reservoir is filling up with wet weather flow during a storm, wet weather flow enters the flush reservoir through the ingress and egress port in the flush reservoir. As the liquid level rises in the flush reservoir, positive pressure automatically opens the air release valve allowing air to purge from the flush reservoir. When the flush reservoir is full, the air release valve automatically closes.
During draining of the sewer or storage tank (e.g. after a storm), a vacuum is created in the air space of the flush reservoir which holds the liquid up in the flush reservoir. When liquid in the sewer or storage tank is drained to a predetermined level (below the elevation of the air intake conduit opening), air is drawn into the flush reservoir via the air intake conduit, breaking the vacuum inside the flush reservoir. Thus, liquid in the flush reservoir is quickly released through the ingress and egress port to the downstream storage tank or sewer resuspending the settled sewer solids and transporting them to a sediment pit for final disposal.